Continuous process for solid phase polymerisation of polyesters

ABSTRACT

A process for the solid phase continuous polymerization of polyester in order to achieve a molecular weight increase, measurable by the intrinsic viscosity IV increase of the polyester, wherein the use of at least a reactor ( 15 ) is provided, the reactor ( 15 ) being cylindrical, rotary around its own central axis (S), substantially horizontal, slightly inclined so as to produce the polymerization of the polyester granules passing through the reactor by gravity thanks to the inclination and the rotation of the reactor ( 15 ), inside the reactor there being produced a purge gas flow having the same or the opposite direction with respect to the flow of the polyester granules.

BACKGROUND

A. Field

The invention relates to a process for the solid phase continuouspolymerisation of polyester. More exactly, the invention relates to aprocess for the solid phase continuous polymerisation of polyester inorder to increase its molecular weight.

B. Related Art

It is known that the molecular weight of a polyester can be measured bythe measure of its intrinsic viscosity IV (“Intrinsic Viscosity”).

It is also known that the molecular weight increase of a polyester canbe achieved by subjecting low molecular weight polyesters, generally ingranules or chips form, to a solid phase polymerisation process that canbe carried out in a continuous moving-bed or in a static-bed (so calledbecause the polymer bed is not fluidised).

Moving-bed or static-bed solid-phase polymerisation processes,particularly intended for the polyethylene terephthalate, whereintemperatures comprised in the range 180-245° C. are applied, are known,for instance, from U.S. Pat. No. 3,405,098, U.S. Pat. No. 4,064,112,U.S. Pat. No. 4,161,578, U.S. Pat. No. 4,223,128, U.S. Pat. No.4,238,593, U.S. Pat. No. 5,408,035, U.S. Pat. No. 5,536,810, U.S. Pat.No. 5,590,479, U.S. Pat. No. 5,708,124 and EP 0,222,714.

According to the teaching of the above mentioned documents, the solidphase polymerisation is preceded by a crystallisation step that can beperformed at a lower temperature (see, for instance, U.S. Pat. No.3,405,098, U.S. Pat. No. 4,161,578 and U.S. Pat. No. 4,223,128), at thesame temperature (see, for instance, EP 222,714) or at a highertemperature (see, for instance, U.S. Pat. No. 4,064,112) with respect tothat applied in the following polymerisation thermal treatment.

The purpose of the crystallisation step prior to the solid phasepolymerisation is to prevent the sticking of the granules during thepolymerisation process, especially at the highest temperatures.

As a matter of fact, it is known that in industrial solid-phasepolymerisation plants sticking phenomena and solid agglomeration of thepolyester granules happen frequently.

This problem is particularly evident when the polyester used as rawmaterial in the polymerisation plant is substituted with a differentpolyester having different needs for the molecular weight increase. Thishappens for example during solid phase continuous moving-bedpolymerisation in producing PET for beverage bottles wherepolymerisation is carried out at temperatures above the amorphouspolyester (prepolymer) glass transition temperature, but below themelting point.

If we analyse all conventional solid phase polymerisation processesavailable today, it will result that the polyester prepolymer(crystallized or partially crystallized) is fed into the top of avertical moving or static bed reactor for solid phase polymerisation inwhich it moves down by gravity in contact with a stream of preheatedpurge gas.

According to known prior art, the purge gas primarily functions to carryoff unwanted by-products such as glycols, water and acetaldehyde, whichare generated during polymerisation, while the polyester gradually movestowards the bottom of the vertical reactor.

In general, there are three important requisites that are to be met forcorrect operation of a continuous solid phase polymerisation process.

First, a steady uninterrupted flow of polymer granules must bemaintained. As a consequence, it is highly important that agglomerationor sticking of polymer granules be avoided because they would impede thesmooth flow of granules and make discharge of the product from thereactor difficult, thereby causing the plant control losing.

Secondly, a suitable combination of reactor residence time andtemperature of granules is required to achieve the desired molecularweight, which is measurable, as indicated above, in terms of itsintrinsic viscosity (“IV”). Since reaction rate increases withincreasing temperature, and IV increases with increasing residence time,desired IV can be attained either by using relatively long residencetime with relatively low temperature or relatively short residence timewith relatively high temperature. However, the ideal combination ofreactor residence time and temperature must be chosen taking intoaccount the first of the requisites indicated above, i.e. the need tomaintain a constant flow of polyester granules, thereby avoiding lumpingor sticking of granules.

Third, the flow regime of polyester granules under processing insidesolid-phase polymerisation reactor, must be as close as possible to theideal “plug flow” behaviour, in a way that all polyester granulespassing through the reactor experience the same process conditions forthe same time duration, giving as a consequence narrow molecular weightdistribution in the obtained product, and more generally narrowdistribution of polymerised granules final attributes, which is a keyfactor for the correct realisation of the following steps in processingproduct with increased molecular weight.

As regards the first requisite, that is the need to avoid the stickingof the polyester granules, it is to be said that this phenomenon ismainly affected by temperature, granules size, bed height, velocity offlow of granules within the reactor and polyester-morphology.

The polyester granules, initially moving freely in a moving bed canstick and clot if, for instance, temperature or bed height increase orif rate decreases.

At solid phase polymerisation conditions, polyester is only partiallycrystalline (typically with 25 to 65% crystallinity). As a consequence,such polyester is not a rigid body, but rather, it is leathery andslightly tacky.

Since tackiness of polymer increases with increasing temperature, thesticking tendency of polyester granules also increases with increasingtemperature.

Consider a fixed bed of polyester granules held motionless inside asolid state polymerisation vertical, cylindrical reactor: on theseconditions at polymerisation temperatures and under pressure due to theweight of the polyester bed, granules to be polymerised, creep into oneanother at contact points and, in time, polymer granules will tend toagglomerate and form larger lumps.

The most effective way to prevent lumping is to constantly renew theinter-granular contact areas so that polymer granules do not have achance to creep into one another. This is done by maintaining constantflow of polymer granules at sufficiently high velocity.

Since sticking tendency increases with increasing specific surface area(area per unit mass) or, more precisely, with increasing specificcontact area of polymer granules, it also increases with decreasing sizeof polymer granules.

A reduced granules size contributes to accelerate the polymerisationprocess, on the other hand, however this increases the sticking tendencyof polymer granules. In the presence of small size granules it istherefore required to counteract the higher sticking tendency with areduction in temperature, which, on the other hand, brings the finalvalues of the process rate back to the typical ones for larger sizegranules processed at a higher temperature.

Furthermore, if the particle size is reduced below certain limits,agglomeration occurs practically at any temperature. Typically thatsuitable size of polymer granules for solid state polymerisation isbetween 0.015 to 0.055 grams per granule.

Within a static or moving bed, the compaction pressure a polymer granulecan experience is approximately proportional to the weight of thepolymer granules in the bed which, in turn, is proportional to the bedheight above the granules. Therefore polymer sticking tendency ishighest at the bottom of the bed and lowest at the top. As a result,lumping of polymer granules usually starts near the bottom of the bed.For this reason there is a practical limit on the bed height of a solidphase polymerisation reactor. At sufficiently high flow velocity,polymer granules change their positions relative to each other (bysliding, for example), and are thereby prevented from forming lumps.Since the rate of changes of contact areas of polymer granules and thereduction in the bulk density of the bed increases with increasinggranule velocity, polymer sticking tendency within the reactor decreaseswith increasing granule velocity. For every combination of reactortemperature, bed height, and particle size, there exists a minimumgranule velocity necessary to prevent sticking. For any given size andshape of polymer granule, the minimum velocity for avoiding stickingincreases with increasing temperature and bed height. Thus a highervelocity is required for a higher polymerisation temperature or greaterbed height.

For instance, for a pilot scale moving-bed vertical cylindrical reactoraccording to the known prior art, which is usually no more than 5 metershigh, granule velocity of less than 0.3 meter per hour can be usedwithout polymer sticking. On the other hand, for commercial scalevertical reactors, with output for instance up to 300 metric ton/day andwhich are conventionally 18 to 22 meters high, a granule velocity of atleast 2 meters per hour is generally required.

A well designed commercial scale solid phase polymerisation plant mustbe capable of continuously producing products of desired IV incompliance with the required specification at a sufficiently highthroughput.

The currently used plants (i.e. Buehler, UOP-Sinco, Hosokawa-Bepex,Zimmer) use single or multiple vertical cylindrical reactors 10 to 30meters in height. In those plants the reactor is operated at atemperature between 200° C. and 230° C. and a polyester granules movingvelocity of 1.00 to 2.52 meters per hour. Within these ranges oftemperature, bed height, and granule velocity, a most suitablecombination of the three variables is chosen to produce product with thedesired IV. Said conventional plants, today available, are capable ofproducing polyethylene terephthalate (PET) resin with an IV between 0.72to 0.86 dl/g, using PET prepolymer with an IV between 0.55 to 0.65 dl/g.Said conventional plants can increase polymer IV by about 0.12 to 0.25dl/g.

For some specific applications, e.g., PET with IV between 0.95 and 1.05dl/g for manufacturing technical/commercial articles (luggage, cords,conveyor belts, etc.) or for tyre cords using PET prepolymer with atypical IV in the range between 0.55 and 0.65 dl/g, or for standardbottle applications where the initial IV of the prepolymer is 0.25-0.45dl/g, it is however, necessary to increase IV by more than 0.25 dl/g.This can hardly be achieved and it often cannot be achieved in aconventional plant using vertical reactors.

In a conventional process, there are two ways to raise the product IV;namely, increasing the reactor temperature or increasing the reactorresidence time of granules. The reactor residence time is constrained bybed height and granule velocity. It can be increased by eitherincreasing the bed height or by decreasing granule velocity. Increasingthe reactor diameter allows an increase in the throughput rate but notin residence time at constant granule velocity. On the other hand, ifreactor temperature is raised to increase the end product IV, polymersticking tendency will therefore increase. To prevent polymer sticking,bed height must be decreased or granule velocity increased. However,either measure reduces reactor residence time and offsets the effect ofthe temperature increase. Alternatively, increasing the reactorresidence time either by increasing the bed height (assuming there is asufficient reactor height) or by reducing the granule velocity resultsin increased polymer sticking tendency.

To prevent sticking, the reactor temperature must be lowered, againoffsetting the effect of the increased residence time on the product IV.

These constraints limit the ability of conventional plants usingvertical single or multiple reactors, to increase intrinsic polymer IV.

Similar situation it is encountered when an industrial scale plant withcapacity above 360 metric tons per day has to be designed forconventional continuous solid phase polymerisation processes.

In fact, in a conventional process, there are two ways to reach highplant production capacity: again by increasing the reactor temperatureor by increasing the product volume (“hold-up”) in the reactor. As faras drawbacks due to the temperature increase are concerned, the sameabove described issues have to be considered. On the other hand, theproduct volume (“hold-up”) of polyester granules in the reactor isconstrained by bed height, reactor diameter and granule velocity. If theproduct volume (“hold-up”) is increased by either increasing bed heightor reactor diameter, or by decreasing granule velocity, polymer stickingtendency will increase. Thus, these constraints limit the maximumcapacity of conventional solid phase polymerisation processes, which useone or more vertical cylindrical reactors.

Nowadays, growing polyester and PET demand has given rise to a need forsolid-phase polymerisation processes by means of which it is possible toachieve a higher increase of polyester molecular weight and a higherproduction capacity, typically >300 metric tons/day on single plant.

The purpose of the present invention is therefore to provide a solidphase polymerisation process of polyester that allows to overcome thelimitations of the processes known so far by permitting to achievebetter results in term of increased intrinsic viscosity of thepolyester.

A further purpose of the invention is therefore to provide a solid phasepolymerisation process of polyester that allows to achieve higherproduction capacities.

In the solid phase polymerisation plants also the purge gas flow ratehas to be just sufficient to effectively remove the reactionby-products. As a matter of fact, a gas excess results in higher costsboth for its supply and for its regeneration and disposal.

Therefore, a further purpose of the invention is therefore to provide asolid phase polymerisation process of polyester that allows to reducethe costs due to the purge gas employment.

These and other purposes are achieved with the process according to theinvention, as claimed in the attached claims.

SUMMARY OF THE INVENTION

Advantageously, the process according to the invention allows to achievehigher molecular weight increases of the treated polyester when comparedwith the ones achievable with the conventional processes of the knownprior art, furthermore avoiding unwanted agglomeration phenomena andother side effects.

The process according to the invention further allows to achieve a highdegree of plug flow (“plug flow”) and, consequently a high homogeneityand uniformity of the final product.

Advantageously, moreover, the process according to the invention allowsto achieve higher production capacities when compared with the plantsexploiting the conventional processes. Always according to the inventionis furthermore possible to advantageously achieve a reduction of energyconsumption, thanks to the decreased ΔP required for the purge gas withrespect to conventional processes.

The invention will be now described more in detail with particularreference to the attached drawing, provided by way of not limitingexample, and illustrating a flow chart of the process according to theinvention.

DESCRIPTION OF THE DRAWING

With reference to the attached FIGURE, a flow chart of a solid phasepolymerisation process according to the invention is schematicallyrepresented.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Polyester prepolymer granules stored in a hopper 11 are fed from here toa crystalliser 13 where they are heated up to a suitable temperature tocause the crystallisation of the granules without sticking.

Preferably, the crystallisation step is carried out in a fluidised bedcrystalliser 13 by utilising a gas flow rate sufficient to cause thepolyester granules to be fluidised with or without mechanical vibration.To this purpose inert gas or air can be used. Crystallization cangenerally be accomplished at residence times in the range of about 2 toabout 20 minutes and, preferably, from about 10 to about 15 minutes. Inthe case of polyethylene terephthalate resin, heating is achieved byfluidising medium (either air or inert gas) at temperatures in the rangeof about 140° C. to about 235° C. and preferably in the range of about200° C. to about 225° C. Residence time to crystallize the polyestergranules to the desired level depends on the crystallizationtemperature; low crystallization temperature requires longercrystallization time.

In general, polyethylene terephthalate prepolymer is crystallized to acrystallization degree corresponding to a density of at least about 1.37g/cm³. The polyester granules can also be crystallised by vaportreatment (see for example U.S. Pat. No. 4,644,049) or by high frequencyenergy field ranging from about 20 to about 300 MHz (see for exampleU.S. Pat. No. 4,254,253).

After being crystallised, granules may optionally be fed into apreheater using purge inert gas. The crystallised polyester granules canoptionally be dried after exiting the crystalliser. However, drying itis not strictly necessary and it is less costly to polymerise “wet”polyester, as it is known from U.S. Pat. No. 3,718,621.

After crystallisation the polyester granules are solid-phasepolymerised.

According to the invention at least one or first cylindrical, horizontalinclined, rotating around the central cylinder S axis reactor similar toa “rotary kiln” may be used, which will be hereinafter for simplicityabbreviated as “HCIRR” referenced in the FIGURE as 15.

Also according to the invention, said at least one or first HCIRRreactor can be one HCIRR reactor of a series of HCIRR reactors, i.e.when the plant has a plurality of HCIRR reactors in series. Alsoaccording to the invention, said single or multiple HCIRR reactor/s canbe provided upstream and/or downstream of conventional verticalreactor/s in a so-called “mixed configuration”, i.e. when the plant hasa plurality of both conventional vertical and HCIRR reactors in series.

Owing to the configuration of the HCIRR reactors and in particular owingto their inclination by an a angle in the range 0.1° and 12°, preferablybetween 1° and 6° with respect to the horizontal line, the maximumpolyester granules bed height is 4-5 meters. This in turn means very lowcompaction pressure if compared to conventional reactors, achieving thusthe most important condition required to obtain either high molecularweight PET or to obtain high output in terms of metric tons per day in asingle line solid phase polymerisation plant. Advantageously, thecombination of the inclination and the rotation, preferably with a speedbetween 0.1 and 10 rpm of the HCIRR reactor 15 ensures proper flow fromone end to the other of the reactor HCIRR 15 and provides to constantlyrenew the inter-granular contact areas so that polyester granules do nothave a chance to creep into one another.

As the weight itself of the granules mass inside the reactor can not beignored with respect to other forces acting as, for example the force ofinertia, preferably the design and operating parameters of the reactorHCIRR 15 will be chosen so that the granules flow regime inside thereactor is characterised by a Froude Number Fr=(ω²·R/g) comprised in therange of 1·10⁻⁴-0.5; where ω is the angular velocity of the reactor; Ris the internal radius of the reactor and g is the gravityacceleration=9.806 m/s².

This flow regime, named “rolling”, is such that, when granules aresubmersed in the bed of solid, they behave as a rigid body and rotate atthe same rotational speed of the HCIRR reactor, and, when they come atthe surface of the solid bed, they slide on the surface itself. Thissolid flow regime is essential in order to have a true “plug flow”behaviour of the solid phase.

The crystallised (or crystallised and preheated) polyester granules arepassed into the top of the only HCIRR reactor 15 (or in the first HCIRRreactor of a series of reactors HCIRR, when the plant has a plurality ofHCIRR reactors in series) and pass through the HCIRR reactor (or thereactors) thanks to its inclination as well as its rotation.

The granule flow rate through the HCIRR reactor 15 is controlled byregulating discharge from the HCIRR reactor itself.

Polymerisation is conducted in a stream of purge inert gas. Purge, asflow, well below the turbulent is generally preferred so to preventfluidisation and entrainment of polyester granules. Furthermore, whenmore HCIRR reactors are present in series the inert gas flow rate willnormally be approximately equal. In said latter case, it is preferredthat the rate in each HCIRR reactor not exceed 1.25 times the rate inany other reactor in a reactor series.

Preferably, furthermore, both in the case of a single HCIRR reactor or aHCIRR reactor series the purge gas passes through the HCIRR reactor 15counter-current to the polyester granules flow direction. Although alsoa co-current with the granules direction inert gas flow can be used,this latter configuration proves to be less efficient and generallyrequires a higher gas flow rate.

The HCIRR reactor 15 can be of any design, with or without internalbaffles, with or without external heating/cooling “jacket”, with orwithout internal heating/cooling coil, that will allow the polyestergranules to be maintained at the desired temperature and residence timeto allow for removal of reaction by-products, as glycol, water andacetaldehyde.

Suitable purge gases for use in the process of this invention preferablyinclude nitrogen, but also carbon dioxide, helium, argon, neon, krypton,xenon, air and certain industrial waste gases and combinations ormixtures thereof can be chosen.

Moreover, optionally, purge inert gas can be recycled to the reactor,after having been purified of organic impurities, preferably untilreaching a level of organic impurities≦100 p.p.m. by weight (CH₄equivalent).

In general the polymerisation temperature will be included in the rangefrom just above the threshold polymerisation temperature to atemperature within a few degrees of the polymer sticking temperature(which may be well below the melting point of the polymer).

For example, when polymerising PET homopolymers, a temperature in theHCIRR reactor (or in the first HCIRR of the series, when a plurality ofreactors is provided) within the range of about 170° C. to about 235° C.and preferably in the range of about 190° C. to about 225° C. issuitable. Temperatures about 205° C. to about 220° C. are preferred.

Modified PET copolymers containing from about 1 to about 3 mole percentisophthalic acid, a percentage based on total acids, are polymerised atabout 5 to 8° C. lower temperatures. Such copolyesters are lesscrystalline and have a great attitude to stick at polymerisationtemperatures.

After exiting the HCIRR reactor 15, the polyester granules, having nowthe required final intrinsic viscosity (0.20-0.30 dl/g higher thanprepolymer one), are cooled in a cooling fluidised bed 17 till reachingtemperature of about 50° C. to about 70° C., for storage and/orpackaging.

In case the process comprises a series of HCIRR reactors, when thepolyester granules exit the first HCIRR of the series, it is preferredto increase their temperature (it is however not to exclude that, atgiven conditions, rather than a temperature increase a temperaturedecrease between a reactor and the subsequent one is required). Thetemperature increase is accomplished in a preheater (accordingly thetemperature decrease, if any, will be accomplished by a cooler). Saidpreheater can be any suitable device, such as a Thermascrew heater, aTorusdisc process, or a fluidised bed heater. It is preferred that thepolyester granules be heated to a temperature that is about 2° C. toabout 20° C. (more preferably about 5 to about 10° C.) higher than thetemperature at which said granules exited the previous reactor. Highertemperatures can be employed in subsequent reactors in the reactorseries. This is possible because of increased crystallinity and highermelting points that occur while the polyester moves through eachsubsequent HCIRR of the series.

After exiting the first HCIRR reactor or after the preheater; thepolyester granules enter the subsequent HCIRR reactor through the topthereof. The second HCIRR reactor can be the same design as the firstHCIRR and be also operated in the same manner as the first HCIRR, exceptfor temperature.

After polymerisation in the second HCIRR reactor, the intrinsicviscosity of the polyester resin will have increased by at least about0.20 dl/g and preferably at least 0.30 dl/g.

Said procedure can be repeated for each HCIRR reactor in a plant with aHCIRR reactor series. It is desirable that the polyester resin exitingfrom the last HCIRR reactor be cooled in a fluidised bed or amechanically agitated heat exchanger. The cooler will cool the resin toa temperature of about 50° C. to about 70° C. for storage and/orpackaging.

This process according to the invention provides greatly increasedresidence time, without increasing polyester granules bed height,thereby enabling higher increase in the polymer molecular weight and ahigher size output in terms of metric tons per day in single line solidphase polymerisation plants, with respect to conventional plants, todayavailable.

Moreover, this process according to the invention allows the use ofprogressively higher temperatures in subsequent HCIRR reactors, whichtherefore increases overall polymerisation rate with a parallel increasein process efficiency.

In conventional solid phase polymerisation processes, which use verticalreactors, the polyester granules being fed to the reactor must have aminimum degree of crystallization in the range of 35-50%, depending onsolid phase polymerisation conditions and granules morphology/dimension,to ensure sticking-free operations.

According to the invention, due to the reduced polymer granules bedheight, thus involving reduced granules compaction pressure, saidminimum degree of crystallization of polyester granules is in the rangeof 0-70%, preferably 10%-30%, reducing therefore the requirement ofcrystallisation in the provided step upstream of the reactor.

Furthermore the invention will be advantageously applicable to anypolyester which can be solid phase polymerised. The most commonpolyesters suitable for use in the invention have at least about 75 molepercent of their acid moieties provided by an aromatic dicarboxylicacid, such as terephthalic acid, isophthalic acid, or a naphthalenicdicarboxylic acid (preferably 2,6-) with the diol moieties provided byglycols such as ethylene glycol, butylene glycol, 1,4-dimethylolcyclohexane and the like or aromatic diols such as hydroquinone andcatechol. Said polyesters can contain other dicarboxylic acids such asadipic acid, isophthalic acid, sebacic acid, and the like. Polyethyleneterephthalate, polyethylene isophthalate, polyethylene naphthalate, andpolybutylene terephthalate homopolymers are representative examples ofsuch polyesters.

Blends of various polyesters can also be solid phase polymerised in theprocess according to the invention. The polyester prepolymers (amorphousstarting polyesters) utilized in this invention can be made in anymanner but are typically prepared by conventional melt phasepolymerisation techniques. These polyester prepolymers have an initialstarting IV of at least about 0.2 dl/g as measured in a 60:40 (byweight): phenol-1,1,2,2,-tetrachloroethane solvent system at atemperature of 30° C. The rate at which polyethylene terephthalateprepolymer can be solid state polymerised also depends on the carboxylend group (i.e. —COOH) content of the prepolymer. Generally, prepolymershaving a carboxyl end group content within the range of about 18% toabout 40% achieve maximum solid state polymerisation rates. It ispreferred for such prepolymers to have a carboxyl end group contentwithin the range of about 24% to 33% (see for example U.S. Pat. No.4,238,593). Suitable polyester prepolymers which can be solid statepolymerized using my invention are comprised of one or more diacidcomponents and one or more diol components. The diacid component in thepolyesters are normally alkyl dicarboxylic acids which contain from 4 to36 carbon atoms, diesters of alkyl dicarboxylic acids which contain from6 to 38 carbon atoms, aryl dicarboxylic acids which contain from 8 to 20carbon atoms, diesters of aryl dicarboxylic acids which contain from 10to 2.2 carbon atoms, alkyl substituted aryl dicarboxylic acids whichcontain from 9 to 22 carbon atoms, or diesters of alkyl substituted aryldicarboxylic acids which contain from 11 to 22 carbon atoms. Thepreferred alkyl dicarboxylic acids will contain from 4 to 12 carbonatoms. Some representative examples of such alkyl dicarboxylic acidsinclude glutaric acid, adipic acid, pimelic acid, and the like. Thepreferred diesters of alkyl dicarboxylic acids will contain from 6 to 12carbon atoms. A representative example of such a diester of an alkyldicarboxylic acid is azelaic acid. The preferred aryl dicarboxylic acidscontain from 8 to 16 carbon atoms. Some representative examples of aryldicarboxylic acids are terephthalic acid, isophthalic acid, andorthophthalic acid. The preferred diesters of aryl dicarboxylic acidscontain from 10 to 18 carbon atoms. Some representative examples ofdiesters of aryl dicarboxylic acids include diethyl terephthalate,diethyl isophthalate, diethyl orthophthalate, dimethyl naphthalate,diethyl naphthalate and the like. The preferred alkyl substituted aryldicarboxylic acids contain from 9 to 16 carbon atoms and the preferreddiesters of alkyl substituted aryl dicarboxylic acids contain from 11 to15 carbon atoms.

The diol component of the polyester prepolymers is normally comprised ofglycols containing from 2 to 12 carbon atoms, glycol ethers containingfrom 4 to 12 carbon atoms, and polyether glycols having the structuralformula HO-(A-O)_(n)—H wherein A is an alkylene group containing from 2to 6 carbon atoms and wherein n is an integer from 2 to 400. Generally,such polyether glycols will have a molecular weight of 400 to about4000. Preferred glycols normally contain from 2 to 8 carbon atoms andmore preferably from 4 to 8 carbon atoms. Some representative examplesof glycols that can be utilized as the diol component include ethyleneglycol, 1,3-propylene glycol, 1,2-propylene glycol,2,2-diethyl-1,3-propane diol, 2,2-dimethyl-1,3-propane diol,2-butyl-1,3-propane diol, 2-ethyl-2-isobutyl-1,3-propane diol,1,3-butane diol, 1,4-butane diol, 1,5-pentane diol, 1,6-hexane diol,2,2,4-trimethyl-1,6-hexane diol, 1,3-cyclohexane dimethanol,1,4-cyclohexane dimethanol, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol. Some representative examples of polyether glycols that can be usedinclude polytetramethylene glycol and polyethylene glycol.

Branched polyester prepolymers can also be solid state polymerised inthe process of the present invention. Such branched polyesters normallycontain branching agents which have three or more functional groups andpreferably three or four functional groups. Reactive functional groupscan be carboxyl groups or aliphatic hydroxyl groups. The branching agentutilized in such branched polyesters can optionally contain bothcarboxyl groups and hydroxyl groups. Examples of acidic branching agentsinclude trimesic acid, trimellitic acid, pyromellitic acid, butanetetracarboxylic acid, naphthalene tricarboxylic acids, andcyclohexane-1,3,5-tricarboxylic acids. Some representative examples ofhydroxyl branching agents (polyols) include glycerin, trimethylolpropane, pentaerythritol, dipentaerythritol, 1,2,6-hexane triol, and1,3,5-trimethylol benzene. Generally, from 0 to 3 percent of a polyolcontaining from 3 to 12 carbon atoms will be used as the branching agent(based upon the total diol component).

High strength polyesters which utilize at least one bis-hydroxyalkylpyromellitic diimide in their diol component can also be solid statepolymerised. The diol component in these polyesters will normallycontain from 5 to 50 mole percent of one or more bis-hydroxyalkylpyromellitic dimides and will preferably be comprised of from 10 to 25mole percent of at least one bis-hydroxyalkyl pyromellitic diimide. Theremaining portion of the diol component is comprised of additionalcopolymerizable diols.

Embodiment 1

In a first embodiment of the invention some experiments were conductedin a pilot plant comprising a feed hopper 11, a crystalliser 13, areactor 15 and a cooler 17.

The crystalliser 13 and cooler 17 were fluid beds using respectivelynitrogen and air as heat transfer and fluidisation medium.

The used reactor 15 is a HCIRR horizontal cylindrical inclined reactorof 1.0 meter inside diameter, 13 meters length, 2° of inclination withrespect top the horizontal plane and rotating at a 1.4 rpm speed. Aspurge inert gas nitrogen was used flowing downwards counter-currentlyrespect to polyester granules flow direction. Gas circuits of fluid bedcrystalliser, of reactor 15 and of fluid bed cooler 17 were isolated bymeans of rotary “interlocks”.

A polycondensation test in the solid phase was carried out usingpolyethylene terephtalate granules containing a percentage by weight ofisophthalic acid of 2.0% and a normal DTA melting point of 253.0° C.Flow rate of polyester granules was 500 kg/h. The ratio between the massflow rate of the purge gas passing through the reactor and mass ofpolyester granules was=0.95. The starting intrinsic viscosity was 0.60dl/g. The acetaldehyde content was 75 p.p.m. Polyester granulestemperature at the reactor inlet, as well as inside the reactor, was212° C. and their average degree of crystallisation X_(c) in the rangeof 39-40%. The polymer obtained had a final intrinsic viscosity of 0.82dl/g keeping constant the temperature of the reactor. The averageresidence time of polyester granules in the reactor was 12 hours.

Embodiment 2

For this second example of the embodiment of the process according tothe invention, all parameters of the test of the first embodiment werekept constant, except polyester granules temperature inside HCIRRreactor. The following four temperatures were used: 214°±0.5° C.,217°±0.5° C., 220°±0.5° C. and 223°±0.5° C. In all those tests theaverage residence time of polyethylene terephtalate granules insideHCIRR reactor has been equal to 12 hours. The final intrinsic viscosityof the solid phase polymerised PET was respectively: 0.839 dl/g, 0.866dl/g, 0.895 dl/g and 0.932 dl/g.

In all four tests a steady uninterrupted flow of polymer granules waskept, with no sign of agglomeration or sticking of granules.

The same test was conducted on a conventional vertical cylindrical“moving bed” reactor with inside diameter=1.6 meters, bed height=8meters, granules velocity=0.32 meters per hour. Same polyethyleneterephtalate granules were employed, with same degree of crystallinityat reactor inlet.

The test run with reactor temperature equal to 216°±0.5° C. wassuccessful and, in 12 hours of residence time of granules inside theprior art vertical “moving bed” reactor, it produced a product withfinal intrinsic viscosity=0.837 dl/g.

The test run with reactor temperature equal to 217°±0.5° C. sufferedbeginning of polyester granules agglomeration phenomena and appeared tobe the limit of the system; a product with final intrinsicviscosity=0.858 dl/g was produced in a residence time of 12 hours,however high product non-uniformity was observed.

As soon as the temperature was increased to perform the test withreactor temperature equal to 220°±0.5° C., “marble-size” lumps werecoming out the reactor, a positive sign of polymer sticking.

Therefore, it was apparent that the maximum allowable reactortemperature had been reached. The reactor temperature was reduced to216° to prevent further polymer agglomeration and to run out the smalllumps already formed inside the reactor. After 14 hours of operation at216° C., the polymer lumps had completely disappeared from the reactorproduct and the product intrinsic viscosity stabilized at about 0.851dl/g.

This illustrated embodiment clearly shows that conventional, vertical,cylindrical, “moving bed” reactor presents a maximum allowable reactortemperature and maximum attainable intrinsic viscosity with a fixedreactor, bed height and bed velocity. In this specific case, the maximumallowable reactor temperature was about 216° C. and the maximumattainable intrinsic viscosity was about 0.851 dl/g with a reactor bedheight of 8 meters, a velocity of 0.32 meters per hour, using aprepolymed with an intrinsic viscosity of 0.60 dl/g.

From the above it will result evident that the process according to theinvention allows to achieve a higher increase of the molecular weight ofpolyester as well as to operate at a temperature well above the onesused before with the conventional moving-bed processes, without stickingand other unwanted effects.

1. A process for the solid phase continuous polymerisation ofpolyesters, comprising the steps of: preparing a mass of polyesterprepolymer granules comprising at least one polyester; feeding saidpolyester prepolymer granules to a crystallizer and heating them to atemperature of about 140° C. to about 235° C. to cause thecrystallization of the granules; feeding said crystallized granules intoa generally horizontal, cylindrical, heated, first rotating reactor,said first reactor being slightly inclined downwardly from a feeding endthereof; producing a purge gas flow inside said first reactor toincrease the intrinsic viscosity of said at least one polyester.
 2. Theprocess according to claim 1, wherein the polyester granules fed intosaid first reactor have a temperature in the range of 185-225° C.
 3. Theprocess according to claim 1, wherein the polyester granules fed intosaid first reactor have a temperature in the range of 180-230° C.
 4. Theprocess according to claim 1, wherein the polyester granules fed intosaid first reactor have a crystallisation degree (X_(c)) greater than10%.
 5. The process according to claim 1, wherein the polyester granulesfed into said first reactor have a crystallisation degree (X_(c))greater than 20%.
 6. The process according to claim 1, wherein thepolyester granules fed into said first reactor have a minimumcrystallisation degree (X_(c)) in the range of 0-70%.
 7. The processaccording to claim 1, wherein said purge gas is an inert gas or air. 8.The process according to claim 1, wherein said purge gas is air with adew point less than −30° C.
 9. The process according to claim 1, whereinthe purge gas is a mixture of gases chosen from the group consisting ofnitrogen, noble gases, carbon dioxide, carbon monoxide and oxygen, andwherein the oxygen content is less than 10% by weight.
 10. The processaccording to claim 1, wherein said purge gas is a mixture of gaseschosen from the group consisting of nitrogen, noble gases, carbondioxide, carbon monoxide and oxygen, and wherein the oxygen content isless than 6% by weight.
 11. The process according to claim 1, whereinthe purge gas has been purified of organic impurities to a level lessthan or equal to 100 p.p.m, by weight (CH₄ equivalent) and is thenrecycled to the first reactor.
 12. The process according to claim 1,wherein said at least one polyester is polyester having at least about75% of its acid moieties provided by terephthalic acid.
 13. The processaccording to claim 12, wherein the polyester has an IPA (IsophthalicAcid) content in the range of 1-20%.
 14. The process according to claim12, wherein the granules of polyester fed into said first reactor havean intrinsic viscosity in the range between 0.55 and 0.65 dl/g.
 15. Theprocess according to claim 12, wherein the granules of polyester fedinto said first reactor have an intrinsic viscosity in the range between0.25 and 0.75 dl/g.
 16. The process according to claim 1, wherein saidat least one polyester is PEN polyethylene naphthalate.
 17. The processaccording to claim 1, wherein said at least one polyester is PBTpolybutylene terephthalate.
 18. The process according to claim 1,wherein the granules fed in the first reactor have a carboxyl end groupscontent in the range of 10-45%.
 19. The process according to claim 1,wherein the granules are cube-shaped with volumes between 1 mm³ and 125mm³.
 20. The process according to claim 1, wherein the granules arespherical with a diameter between 1 mm and 5 mm.
 21. The processaccording to claim 1, wherein the granules are extended cylinders oflength less than 10 mm and circular or square cross-section having,respectively, a diameter or side less than 5 mm.
 22. The processaccording to claim 1, wherein the polyester granules are pancake-likeplatelets of diameter less than 3 mm and thickness less than 3 mm. 23.The process according to claim 1, wherein the polyester granules have anirregular shape with a volume between 1 and 125 mm³.
 24. The processaccording to claim 1, wherein the mass of prepolymer crystallisedgranules is achieved by subjecting the polyester granules to acrystallisation step in a fluidised-bed crystallizer having at least onebed, said bed being fluidised by means of a gas flow sufficient togenerate the fluidisation of the polyester granules with or withoutmechanical vibration.
 25. The process according to claim 24, whereinsaid gases employed for the crystallisation are inert gases or air. 26.The process according to claim 24, wherein said crystallisation step isperformed with a residence time selected from the group consisting ofbetween 2 and 20 minutes and 10 to 15 minutes.
 27. The process accordingto claim 1, wherein the granules are heated to cause the crystallisationup to temperatures between 200-225° C.
 28. The process according toclaim 1, wherein the polyester granules inside said first reactor aresubjected to at least one of a solid phase polycondensation, drying, anddealdehydisation.
 29. The process according to claim 1, wherein theintrinsic viscosity of the polyester is increased at least 0.35 dl/g.30. The process according to claim 12, wherein the intrinsic viscosityof the polyester is increased at least 0.4 dl/g.
 31. The process ofclaim 1, which allows a high degree of plug flow to yield highuniformity and homogeneity of the final product.
 32. The process ofclaim 31 wherein the rotating reactor is at an angle of 0.1 to 12degrees to the horizon and rotates at a speed of 0.1 to 10 rpm.
 33. Theprocess of claim 24 wherein the rotating reactor is at an angle of 0.1to 12 degrees to the horizon and rotates at a speed of 0.1 to 10 rpm.34. The process of claim 33, wherein the granules form a bed of granulesin the rotating reactor with a maximum height of 4 to 5 meters.
 35. Theprocess of claim 34, wherein the granules internal of the bed behave asa rigid body and rotate at the same rate as the rotating reactor andwhen said particles are at the surface of the bed, slide at the surfaceand are subject to an inert purge gas flowing in a direction countercurrent to the flow of the granules.
 36. The process of claim 35, whichcontain multiple reactors in series and in which the purge gas flow ratein any one reactor does not exceed the purge gas flow rate in any otherreactor by 1.25 times.
 37. The process of claim 36, wherein thetemperature in the first reactor is from 205° C. to 220° C. and thetemperature is progressively higher downstream in a second or furtherreactor.
 38. The process of claim 37, which has a production output ofgreater than 300 metric tons per day of polyethylene terephthalate forbeverage bottles.
 39. The process of claim 34, wherein the degree ofcrystallization is from 10-30% when the granules enter the firstrotating reactor.
 40. The process of claim 2 further comprising the stepof forming beverage bottles from said granules from said rotatingreactor.